Engines may use a turbocharger to improve engine torque/power output density. In one example, a turbocharger may include a compressor and a turbine connected by a drive shaft, where the turbine is coupled to the exhaust manifold side and the compressor is coupled to the intake manifold side. In this way, the exhaust-driven turbine supplies energy to the compressor to increase the flow of air into the engine.
The compressor is intended to work in an operating range between two conditions, surge and choke. Surge occurs during low air mass flow, when the air flow through the compressor stalls and may reverse. The reversal of air flow may cause the engine to lose power. One source of surge, tip-out surge, may occur when the engine suddenly decelerates. During tip-out surge, the engine and the air flow mass through the compressor may slow down, while the turbocharger continues to spin due to inertia and delays through the exhaust system. The spinning compressor and low air flow rate may cause rapid pressure build-up on the compressor outlet, while the lagging higher exhaust flow rate may cause pressure reduction on the turbine side. When forward flow through the compressor can no longer be sustainable, a momentary flow reversal occurs, and the compressor is in surge.
A second source of surge may be caused in part by high levels of cooled exhaust gas recirculation (EGR). EGR may be used for reducing NOx emissions from diesel engines and for controlling knock in gasoline engines. High levels of EGR may increase compressor pressure while decreasing mass flow through the compressor causing the compressor to operate inefficiently or in the surge region.
Choke occurs when the air flow mass flowing through the compressor cannot be increased for a given speed of the compressor. During choke, the turbocharger cannot provide additional air to the engine, and so the engine power output density cannot be increased.
Therefore, it can be desirable to increase the operating range of the compressor and the turbocharger by reducing the air flow rate before surge occurs and increasing the air flow rate before choke occurs. One solution that has been used to widen the operating point is a passive casing treatment. The passive casing treatment includes a pair of immovable slots that modify the air flow through the compressor. During low air mass flow conditions, the slots of the passive casing treatment may provide a path to recirculate partially pressurized air back to the compressor inlet. The recirculated air flowing through the compressor may enable less air to flow through the compressor before surge occurs. During high air mass flow conditions, the slots of the passive casing treatment may provide a path to short-circuit air flow through the compressor so that the choke occurs at a higher air mass flow rate.
However, the inventors herein have recognized that an effective location for a passive recirculation port to prevent surge is different from an effective location for a port to prevent choke. In the present disclosure a continuously open port to prevent surge is disclosed as is a separate port to prevent choke, the inlet of which may be opened or closed to airflow through a compressor inlet. A bleed port may be arranged such that its inlet is at a height below the full blades of a compressor impeller but above the splitter blades. Conversely, the choke port may serve to provide air to the base of the impeller, below the splitter and turbocharger blades. Opening an inlet of the choke port may furthermore be variable, such that an engine controller may control when, and the extent to which, the inlet of the choke port is exposed to airflow from the compressor inlet. Such a signal may be provided when a compressor is at near choke conditions. When the inlet of the choke port is open, air may be drawn into the compressor at its base and may serve to effectively extend the compressor flow capacity.
A compressor casing is disclosed herein that may have a pair of annular disks located at the periphery of the interior of compressor downstream of the inlet. The annular disks comprise alignable openings around their circumference, these choke slots may be aligned to open into an inlet of a choke port allowing air to be drawn into the base of the impeller. Furthermore, when not overlapping, the choke slots of the two annular disks effectively cut off the inlet of the choke port from air flow from the compressor inlet.
Systems and methods are disclosed for a turbocharger compressor, the system comprising: an actuatable annular disk comprising choke slots therein; an outer annular disk comprising choke slots therein; and an actuator to rotate the actuatable annular disk relative to the outer annular disk to vary alignment of the choke slots of the actuatable annular disk and the outer annular disk. The actuator may be controlled by an engine controller responsive to operating conditions of the compressor and actuated to align choke slots. Alignment of the choke slots allows air to be drawn into the impeller effectively expanding the compressor flow capacity to prevent compressor choke.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.